1. Field of the Invention
The present invention relates to an image detection system provided with a plurality of area sensors for use, for example, in various types of cameras, binoculars, and optical measurement instruments.
2. Description of the Prior Art
It is a well-known technique to use an image detection system provided with a pair of image sensors in order to detect the distance of a measurement object or detect the focus condition of an objective optical system. This technique is used to achieve automatic focusing, that is, to bring an objective optical system into an in-focus condition automatically in accordance with the result of distance or focus detection. In the following descriptions, such distance detection or focus detection will be referred to as automatic focusing.
An image detection system having a pair of image sensors achieves automatic focusing in the following manner. The light from an object is split into two light beams by means of a separator lens, and the two light beams are individually projected onto the pair of image sensors. Then, on the basis of the two images formed on the two image sensors, the distance of the object, or the amount of an image blur caused by a camera shake (hereafter referred simply as an "image blur"), is calculated. Accordingly, it is necessary to read out signals from both image sensors within a single integration operation.
This can be realized, of course, by providing each image sensor with a separate driving circuit and a separate signal processing circuit. However, in consideration of cost reduction, some conventional image detection systems are provided with only one driving circuit and one signal processing circuit and are designed to achieve, nevertheless, automatic focusing by reading out signals from two image sensors in a single integration operation.
This type of image detection system has as its pair of image sensors a pair of line sensors. In the image detection system, only the electric charges appearing in the sensor that is being read at a particular moment are transferred to shift registers for output, whereas the electric charges appearing in the other sensors are held in an accumulation area provided between the pixels and the shift registers. When reading of one sensor is complete, the next sensor is read by transferring the electric charges held in the accumulation area for that sensor to the shift registers for output.
However, this conventional construction is based on line sensors, and therefore, if it is applied to image detection systems based on area sensors, like those embodying the present invention, the resulting systems will suffer from the following disadvantages. First, providing electric charge accumulation areas in area sensors result in reduced aperture ratios of the area sensors. Second, an image detection system based on line sensors is not ready for image blur detection, and therefore it is not provided with any means for reading a signal from only one of a given pair of sensors, which means that it is not possible to use a single image detection system both for automatic focusing and for image blur detection.